The Bridge Troll is a special monster. A single level 5 Bridge Troll appears in the Havendale Bridge dungeon, blocking the bridge between the two halves on the dungeon. Initially Trait: Pacifist Pacifist, when you talk to it, it will offer to let you pass for 20 gold. Agreeing to pay 20 gold will remove the Bridge Troll from the dungeon. If you choose to fight it instead, it will lose the Trait: Pacifist Pacifist trait and can be fought normally.
Due to its high resistances, the Bridge Troll is much stronger than a normal level 5 monster. Fortunately, there are ways to get it out of the way without fighting it or paying it. It can be pushed out of the way with PISORF or Knockback, teleported out of the way with WEYTWUT, or even petrified with IMAWAL so that its statue can be destroyed with ENDISWAL.
The Item: Slayer's Wand Slayer's Wand is not a viable option, as using it on the Bridge Troll causes it to respawn as a level 10 Bridge Troll, with appropriately inflated stats. This does give you a second Sensation Stone as both Bridge Trolls drop one, but this is unlikely to be worth the cost.
The Bridge Troll also appears as a miniboss in the Tower of Gaan-Telet and Vicious Gaan-Telet. Here, it lacks the initial Trait: Pacifist Pacifist trait and there is no option other than to fight it, but it still drops the Sensation Stone.